Iron
[Ar] 3d⁶ 4s² · 2 · 8 · 14 · 2
The metal that named an age and makes up the core of our planet. Iron is the cheapest and most-produced structural material in history, and it is also what carries oxygen in the blood of every vertebrate.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 26
- Atomic mass
- 55.845 а.е.м.
- Electron configuration
- [Ar] 3d⁶ 4s²
- Electrons per shell
- 2 · 8 · 14 · 2
- Block
- d-block
- Group
- Iron group
- Period
- 4
- Electronegativity (Pauling)
- 1.83
- Electronegativity (Allen)
- 1.8
- Atomic radius
- 156 pm
- Covalent radius
- 132 pm
- Van der Waals radius
- 194 pm
- Ionisation energy 1
- 762.5 kJ/mol
- Ionisation energy 2
- 1 561.9 kJ/mol
- Ionisation energy 3
- 2 957 kJ/mol
- Ionisation energy 4
- 5 290 kJ/mol
- Ionisation energy 5
- 7 240 kJ/mol
- Electron affinity
- 15.7 kJ/mol
- Common oxidation states
- +2, +3
- Oxidation states
- -4, -2, -1, +1, +2, +3, +4, +5, +6, +7
Physical properties
- State at 25 °C
- solid
- Density
- 7.874 g/cm³
- Melting point
- 1 811 K · 1 537.9 °C
- Boiling point
- 3 134 K · 2 860.9 °C
- Speed of sound
- 5 120 m/s
Thermal properties
- Heat of fusion
- 13.81 kJ/mol
- Heat of vaporisation
- 340 kJ/mol
- Specific heat capacity
- 0.449 J/(g·K)
- Thermal conductivity
- 80.4 W/(m·K)
Mechanical properties
- Young's modulus
- 211 GPa
- Shear modulus
- 82 GPa
- Bulk modulus
- 170 GPa
- Poisson ratio
- 0.29
- Mohs hardness
- 4
- Brinell hardness
- 490 MPa
Electrical and magnetic properties
- Electrical resistivity
- 96.1 nΩ·m
- Magnetic ordering
- ferromagnetic
- Curie point
- 1 043 K
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- body-centred cubic (bcc)
- Lattice constants
- a = 286.65 pm
Abundance
- In the crust
- 56 300 mg/kg
- In the ocean
- 0.002 mg/L
- In the universe
- 1.1·10⁶ mg/kg
- In the human body
- 60 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 54Fe | 5.845 % | stable | — |
| 56Fe | 91.754 % | stable | — |
| 57Fe | 2.119 % | stable | — |
| 58Fe | 0.282 % | stable | — |
| 60Fe | — | 2.6 Myr | β− |
Iron-56 has the highest binding energy per nucleon of any nucleus: past it neither fusion nor fission releases energy, which is why stellar burning ends at iron.
Discovery
- Year of discovery
- known since antiquity
- Discovered by
- known since antiquity
- Where
- no data
- Origin of the name
- Old English iren; the symbol Fe is from Latin ferrum
History
The first iron people used was meteoritic; the Egyptians called it metal from the sky. Smelting from ore began around 1200 BCE in Anatolia and displaced bronze within a few centuries — not because iron is better, but because its ores are incomparably more common.
Where it occurs
The fourth most abundant element in the crust at 5.6 %, and the most abundant in the whole Earth once you count the core. The main ores are haematite and magnetite; the largest deposits formed 2.4 billion years ago when new oxygen precipitated the iron dissolved in the oceans.
How it is produced
The blast furnace: ore, coke and limestone give pig iron, which is then blown with oxygen in a converter to burn off excess carbon. Direct reduction with hydrogen is the route to carbon-free steelmaking.
Role in living things
Haemoglobin: one iron atom in a haem binds one oxygen molecule, and four haems per protein carry oxygen from lungs to tissue. Iron deficiency is the most common nutritional deficiency in the world.
Safety
Iron itself is harmless, but acute poisoning from iron supplements is a frequent cause of childhood poisoning. Hereditary haemochromatosis lets iron accumulate and damages the liver and heart.
Uses
- Steel: over 95 % of all metal produced worldwide by mass
- Cast iron for castings and machine beds
- Magnetic cores for transformers and motors
- The catalyst in Haber–Bosch ammonia synthesis
- Pigments: ochre, red lead oxide, iron oxide reds
Curiosities
- The iron-56 nucleus sits at the bottom of the energy curve — which is precisely why massive stars explode once they reach iron.
- Iron's Curie point is 1043 K: above it a magnet stops being a magnet.
- Blood is red and mollusc haemolymph is blue because they carry oxygen on copper rather than iron.
- Meteoritic iron was worked two thousand years before smelting began: the dagger from Tutankhamun's tomb is made of it.
- Earth's magnetic field is generated by convecting liquid iron in the outer core.